When a large number of terminals such as mobile telephones concentrate in a particular area like at a shrine on New Year's Day or when an event is held at a horse racetrack, terminals that are unable to perform communication arise. The number of terminals that can connect to a single base station is limited and when a large number of terminals become subjected to a congested state such as those described, even if the terminals are within an area enabling communication, some terminals are unable to perform communication.
In addition to architecture in which such terminals directly communicate with the base station, techniques that autonomously build communication paths between terminals without passing through a base station have been disclosed. These techniques involve ad hoc communication in which a terminal that cannot directly communicate with the base station performs ad hoc communication with nearby terminals to thereby communicate with the base station via a management terminal that is connected to the base station (see, for example, Japanese Laid-Open Patent Publication No. 2007-89021).
Further, another technique is present in which multiple terminals search for surrounding terminals and based on a given selection criteria, determine terminals as a leader candidate, whereby autonomous grouping in the ad hoc network is performed (see, for example, Japanese Laid-Open Patent Publication No. 2010-45525). According to another technique, when an ad hoc network is built by radio sensors, the base station determines based on a radio sensor list and signal strength, a radio sensor that is to be a parent node and performs grouping for each parent node (see, for example, Japanese Laid-Open Patent Publication No. 2007-243794). Another technique involves a radio sensor network in which various types of radio units are disposed respectively in terminals and at access points to perform communication that is favorable in terms of interference properties, irrespective of the communication environment (see, for example, Japanese Laid-Open Patent Publication No. 2010-183215).
Nonetheless, terminals that cannot directly communicate with the base station communicate with the base station by ad hoc communication. As a result, communicated packets have to pass through a terminal that is directly communicating with the base station, whereby the bulk of communication concentrates at the terminal, increasing the communication load on the terminal. In addition, terminals such as mobile telephones have limited resources, such as power supply and space, and are powerless unlike terminals equipped on vehicles.
With the techniques disclosed in Japanese Laid-Open Patent Publication Nos. 2007-89021, 2010-45525, 2007-243794, and 2010-183215, the ad hoc communication does not consider cases where a large number of terminals concentrate in a particular area. Consequently, the ad hoc communication is performed and when a large number of terminals concentrate in a particular area, the communication load concentrates at a single terminal that is directly communicating with the base station, whereby the terminal fails, unable to tolerate the concentrated communication load.
For example, at the terminal directly communicating with the base station, a radio signal received by a communication module is converted into digital data (packets); the packets are stored to a communication buffer in the memory by the CPU; and thereafter, a communication processing unit analyzes the packets and processes the data in the packets according to the header information. Thus, although the communication packets are processed after accumulating in the buffer, if the rate at which packets are received surpasses the rate at which the packets are processed, the buffer overflows.
For example, packets are assumed to be received at 100 Mbps from another ad hoc communication terminal and the packets are assumed to be transmitted at 20 Mbps to the base station. If the capacity of the communication buffer is 10 Mbytes, the communication buffer overflows in merely 1 second. If the communication buffer overflows, packet loss occurs and the throughput of the network overall drastically drops consequent to retransmission of the packets.
In ad hoc communication, multiple terminals are connected and respectively have communication buffers, the majority of which have sufficient available capacity. However, as described above, a problem arises in that the overflow of the buffer of a single terminal invites drops in the performance of the network overall.